Move hands from two seperate rig from a single controller
Yes, you can.

The two arms should have an IK Chain, you can set a copy location constraint to both the IK targets (Man and child), to follow the empty. You can animate the "influence" parameter of the copy location constraint, so - in the section of the animation in which you want the arms to be controlled by the empty - delete all animation data ot the arms, set two keyframes to get full constraint influence (1.0), and animate the empty as desired (and the pole bones, if present, to set the elbows angles).